widgets: Fix multi-select bug caused by 3e101922

This patch attempts to fix an issue I created with 3e101922 (Diagnosed by @Jehan)

The call to gtk_tree_view_set_cursor () causes the selection to revert to single selection mode; this prevents users from multi-selecting options in lists that support it. To fix this while preventing the original bug fixed in 3e101922, the call to gtk_widget_grab_focus () is made via g_signal_connect_after ().